Crack initiation and propagation in Chromium pre-alloyed PM-steel under cyclic loading

Powder metallurgy processing of steels typically results in materials characterized by residual porosity, whose sizes and morphology, together with the microstructure, strongly affect the fatigue crack growth behaviour of the materials. Prismatic specimens were pressed at 7.0 g/cm3 density from Astaloy CrM powder and sintered under different conditions, varying the sintering temperature and the cooling rate after sintering. Optical observations allowed us to evaluate the sizes and the morphology of the porosity and the microstructural characteristics for all the investigated conditions. Fatigue tests were performed at R-ratio equal to 0.1 to investigate the threshold zone and to calculate the coefficients of the Paris law. All the tests were carried out according to the compliance method, and the crack length was evaluated during the tests. Moreover, KIC tests were performed in order to complete the investigation. On both fatigue and KIC samples, a fractographic analysis was carried out to investigate the crack path and the fracture surface features. The results show that the exponent of the Paris law is about 6.0 for 1120°C sintered and about 4.7 for 1250°C sintered materials. Interesting data have been also found for the threshold values.